Please, when trying to find best Karma XPU performance for a certain price:

• can I rely on any other benchmarks of the cards, that are available online? (redshift, octane)
  
• second GPU of the same kind doubles the performance? Or is it something like 90% of linear increase?
  
• more GPUs probably increase the time needed to load the scene onto the GPU and can slow down the viewport time to first pixel?
  

The only thing that matters is VRAM and that it has nvidia optix support.

That leaves either the Quadro RTX 6000, the RTX A6000, A5000, or the Geforce RTX 3090 and 3090 Ti. The RTX 3090 is probably the best price performance ratio since it costs as much as an A4000 for A6000 performance. The 3090's 24GB can be fairly restricting, you couldn't render a lot of production scenes with it.

You can certainly play around with cards with less vram but even modest scenes will quickly fill it.

Thanks a lot for the info! Fortunately I am creating quite simple motion graphics, so hopefully memory should not be a problem.

I found the XPU diagnostics in the display setting (attached), so I will test some of my scenes and their memory requirements.